Kivy

Kivy runs on Linux, Windows, OS X, Android and iOS. You can run the same code on all supported platforms.
It can use natively most inputs, protocols and devices including WM_Touch, WM_Pen, Mac OS X Trackpad and Magic Mouse, Mtdev, Linux Kernel HID, TUIO. A multi-touch mouse simulator is included.
The framework is stable and has a well documented API, plus a programming guide to help you get started.
The toolkit comes with more than 20 widgets, all highly extensible. Many parts are written in C using Cython, and tested with regression tests.